This invention relates to electronic filters and more particularly to an improved switched-capacitor elliptic state variable filter circuit that is adaptable for implementation as an integrated circuit device.
In my previous U.S. Patent, application, Ser. No. 940,473, filed on Sept. 8, 1978, and now U.S. Pat. No. 4,179,665 an elliptic filter circuit is described using switched capacitors with three operational amplifiers connected in series. This circuit utilizes a feedback connection between the output of the second operational amplifier and the input of the first operational amplifier (which is also the input to the circuit) and a feed forward connection from the input voltage source and the output of the first operational amplifier. These feed forward signals are combined with the output from the second operational amplifier. A plurality of switched capacitors in the circuit for controlling the inputs to the various operational amplifiers are all connected to a two-phase clock driver operated on a preselected frequency. In the transfer function of such a filter circuit it is essential that the transmission zero in the "z" domain be on the unit circle so that there is infinite attenuation at that zero frequency.
With the previous circuit, the transmission zero of the transfer function was forced onto the unit circle by selecting precise capacitor ratios. Thus, if the capacitor values varied from the calculated design specifications and the ratios were off even a small amount, the transmission zero was not exactly on the unit circle but was either inside or outside. This caused a filter frequency response that did not provide infinite attenuation at the zero frequency and the desirable sharp roll-off of the frequency/loss curve. The present invention solves this problem by providing a filter circuit that has infinite attenuation at the zero frequency regardless of the capacitor ratio.
Another problem with prior switched-capacitor filters, particularly when incorporated within large monolithic integrated circuits, was that they tended to be sensitive to parasitic capacitances. So called "stray" capacitors are caused by the junction capacitance of switches and line substrate capacitance created by typical device topology. Since frequency response has a relatively high sensitivity to capacitor ratio errors, it was necessary to make the required circuit capacitors relatively large in order to overcome or reduce the effect of such stray capacitance. Since these stray or parasitic capacitances are mainly junction type capacitances, they are voltage dependent and non-linear and therefore have the added disadvantage of causing harmonic distortion in the filter. The present invention solves these problems by completely eliminating the effects of stray capacitors, thereby allowing the regular circuit capacitors to be made of minimum size and conserving essential chip area while also improving the filter performance by increasing harmonic linearity.